Generally, auxiliary power unit (APU) designs include a flow divider valve to direct the flow of fuel between primary fuel injectors and main fuel injectors. In addition, there is a fuel pressure sensor that monitors the primary fuel pressure supplied to the primary fuel injectors.
During APU starting, the fuel system controller, typically a Full Authority Digital Electronic Control (FADEC), first schedules fuel while it determines when the fuel manifold is filled. While filling the manifold, the fuel system controller also commands the ignitor. The fill schedule is designed to fill quickly before the APU starter accelerates the APU beyond its light-off window. Once the fuel system controller detects that the manifold is filled, the fuel system controller schedules a light-off fuel schedule and monitors engine measured turbine temperature (EGT) in order to detect APU light-off.
The manifold is declared full when the fuel pressure sensor indicates filled pressure. If the flow divider fails in an open condition, fuel pressure will not build up in the manifold and the system will over fuel the APU which may result in a fail to start and excessive fuel collecting in the combustor.
The fuel system controller currently does not detect a failed flow divider. Improvements in determining a failed flow divider valve is therefore needed in the art.